Electrically controlled Fabry-Perot resonators have recently found use in a wide variety of optical devices. See the article "Electrooptic Nonlinear Fabry-Perot Devices", by P. W. Smith et al, IEEE Journal of Quantum Electronics, Vol. QE-14, No. 3, March 1978, pages 207-212. The device disclosed in this Smith et al article can perform a wide variety of functions including optical limiting, pulse shaping, and amplification, and be used as a switching and logic element. It is suggested on page 208 of the Smith et al article that an integrated optics version can be constructed by using an electro optic modulator incorporating a titanium diffused waveguide in a lithium niobate substrate as the resonator medium. To construct such a device requires that the end faces of the titanium diffused waveguide be flat and substantially perpendicular to the direction of propagation in the waveguide. To achieve this flat surface by the standard techniques of cutting and polishing the end of the crystal is extremely difficult since rounding of the edges almost always occurs with polishing, and the titanium diffused waveguide emerges at one edge of the crystal.
It is well known in the art that cleavage planes in the crystal can be utilized to provide reproducable and rectangular ends without difficulty. See for example, the article entitled "Flip-Chip Approach to Endfire Coupling Between Single Mode Optical Fibres and Channel Waveguides", by H. P. Hsu and A. F. Milton, Electronics Letters, Vol. 12, No. 16 pages 404-405, Aug. 5, 1976. Hence, a Fabry-Perot resonator for use in the Smith et al type devices can be constructed by using a lithium niobate crystal with cleaved ends as the parallel end faces of the resonator. Since lithium niobate has an index of refraction that is not radially different from that of air, an external mirror may be required at each of the cleaved ends to provide the necessary degree of reflection.
As pointed out in the Smith et al article, it is highly desirable to have as high a degree of sensitivity as possible in the electrically controllable Fabry-Perot resonator. That is, it is desirable to have a resonator medium which produces as large a phase shift as possible for any given change in voltage across the electrodes of the electrically controllable Fabry-Perot resonator.